ghc_paths.bzl

def _ghc_paths_module_impl(ctx):
  tools = ctx.toolchains["@io_tweag_rules_haskell//haskell:toolchain"].tools
  ghc = tools.ghc
  ctx.actions.run_shell(
      inputs = [ghc],
      outputs = [ctx.outputs.out],
      command = """
      cat > {out} << EOM
module GHC.Paths (
        ghc, ghc_pkg, libdir, docdir
  ) where

libdir, docdir, ghc, ghc_pkg :: FilePath

libdir  = "$({ghc} --print-libdir)"
docdir  = "DOCDIR_IS_NOT_SET"

ghc     = "{ghc}"
ghc_pkg = "{ghc_pkg}"
EOM""".format(
          ghc = ghc.path,
          ghc_pkg = tools.ghc_pkg.path,
          out = ctx.outputs.out.path))
  return [DefaultInfo(runfiles=ctx.runfiles(collect_data=True))]

ghc_paths_module = rule(
    _ghc_paths_module_impl,
    toolchains = ["@io_tweag_rules_haskell//haskell:toolchain"],
    outputs = {"out": "GHC/Paths.hs"},
)